For example from the specialist article “The Role of Sterol Glucosides on Filter Plugging”, Inmok, Lee et al., Biodiesel Magazine, April 2007, it is known that in the transesterification of natural fats or oils with alkyl alcohol the steryl glycosides naturally present in the fats or oils largey lose their solubility both in oil or fat and in the fatty acid alkyl ester obtained by transesterification, so that they are precipitated as fine suspended particles and in use of the fatty acid alkyl ester as biodiesel lead to cloggings of the diesel filters and to deposits in the engine.
In natural fats and oils, the steryl glycosides largely are present in the so-called acylated form, i.e. a fatty acid residue is coupled to their sugar fraction via an ester bond, which provides for the dissolution in fat, oil and fatty acid alkyl ester. In the transesterification, in the presence of the alkyl alcohol and transesterification catalyst, however, the fatty acid residue is separated from the sterol glycoside molecule, so that the solubility gets lost and the above-mentioned disturbing precipitations are obtained.
To remove steryl glycosides from the fatty acid alkyl ester obtained by transesterification, the so-called crude biodiesel, several methods were proposed. As reported in WO 2009/132670, for example, a method exists in which the crude biodiesel is cooled and the precipitated particles then are filtered off. In its execution, however, this method is extremely expensive. WO 2009/132670 ifself proposes the use of an adsorbent consisting of a smectite/silica gel mixed phase.
In the above-mentioned specialist article by Lee, Inmok et al., a filtration of the biodiesel through diatomaceous earth is proposed.
WO 2009/106360 proposes to break down the steryl glycoside molecules into the sterol and the sugar fraction by means of an enzyme.
In this method it is disadvantageous that very special agents, the adsorbent, filter material or the enzyme must be purchased for carrying out the method, and that possibly, after carrying out the method, these auxiliary substances must be disposed of at high cost.
The German Patent Application 10 2008 050935.3-44, not disclosed yet, proposes to vary the acid wash of the crude fatty acid alkyl ester known from DE 10 2006 044 467 B4 such that it is able to also wash out steryl glycosides, beside the other impurities, from the fatty acid alkyl ester phase, the so-called crude biodiesel. During the wash, the steryl glycosides are dissolved out of the ester phase and form a new phase substantially consisting of steryl glycoside/fatty acid alkyl ester/water agglomerates. This process is explicable in that the steryl glycoside molecule consists of a hydrophilic part, the sugar part, and a hydrophobic part, the sterol part.
By intensive treatment of the agglomerate phase with mechanical stirrers its density is increased to such an extent that it is mechanically, e.g. by centrifugation, separable from the ester phase as so-called biodiesel sludge.
According to the prior art, no method exists for reprocessing this sludge, so that the same represents a considerable disposal problem and the valuable substances contained therein get lost.
Therefore, it has been the object to provide a simple method producing as little residues as possible for reprocessing a biodiesel sludge substantially consisting of steryl glycoside/fatty acid alkyl ester/water agglomerates.